In-wheel drives are the lightest, most direct, highest efficiency means of propelling wheeled vehicles. They can be used in vehicles of all sizes, ranging from electric bicycles to transport trucks and commercial jets. One of the first vehicles to use an in-wheel drive was Ferdinand Porsche's prototype—the precursor to the first all-wheel-drive electric car. In the time since Porsche's vehicle was introduced, combustion-powered engines have dominated the passenger vehicle market due to their high power density. With that said, technology has improved in recent decades, leading to advances in materials, control, and design. Several companies have introduced new in-wheel drive systems that take advantage of these advances, but the industry has been hesitant to accept these systems. Industry hesitation has been fuelled by problems that come from three primary sources: technological maturity, vehicle drivability, and battery technology.
Although in-wheel motors have been available for over 100 years, most modern devices are based on very new technology. These new devices are primarily brushless DC and induction motors, and they share many design features, including the use of permanent magnets on the rotor, and copper windings on the stator. Such motors have several innate problems regarding in-wheel applications. The use of permanent magnets increases their cost and reduces their durability, and in some cases the use of an internal rotor also introduces concerns regarding the sealing of their cases.
With regard to the second major source of problems that stand in the way of the in-wheel drive market, the implementation of in-wheel drives in modern vehicles results in a variety of suspension-related safety issues. By attaching a motor to the wheel, the unsprung mass is increased, and thus the response of the suspension system to disturbances is slowed. This phenomenon can be beneficial in circumstances where the tire does not leave the road, but when the tire does, the vehicle can experience a loss of steering control. Such effects are typically not noticed in everyday driving, but some subjective changes to the drivability of the vehicle do occur. Many of these changes can be mitigated by reconfiguring the suspension system to suit the increased unsprung mass, but a vehicle with an increased unsprung mass will always have a reduced suspension capability in comparison to a vehicle with a small unsprung mass.
The final major source of problems for in-wheel motor vehicles is battery technology. Batteries are known to have a poor response to thermal variations; they have been featured in several prominent accidents caused by battery combustion, they are expensive, and they are heavy. Most in-wheel motor developers are focused on mass-market adoption of their systems, but few have considered the specific configuration of the complete drive system that their motor will become a part of. In-wheel motors have thus succumbed to the same issues that have limited the growth of the electric vehicle market.